Hepatocytes will be obtained from a liver biopsy specimen at the time of an elective cholecystectomy to analyze the relative levels of mutant and normal apolipoprotein B mRNA levels in the hepatocytes of a single individual who is heterozygous for familial hypobetalipoproteinemia. The relative rates of synthesis of the truncated (mutant) apo-B protein and the full-length apo-B protein will also be determined by 35S metabolic labeling studies in cultured hepatocytes.